This invention relates to a method of characterizing lithography projection equipment used in manufacturing of integrated circuits.
A conventional optical lithographic stepper system, for imagewise exposure of a coating of resist on a semiconductor wafer, includes a light source for emitting a beam of actinic radiation directed towards the wafer, an imaging lens for imaging the light source on an exposure mask (also called a reticle) that defines a pattern of features that are to be transferred from the mask to the resist coating, and a projection lens for imaging the mask on the resist coating. In many applications, the pattern defined by the exposure mask is binary. In this case, every point of the mask is either opaque or transparent and, subject to diffraction limitations, the lithographic system results in the resist coating being exposed in regions that correspond to transparent regions of the mask and being unexposed in regions that correspond to opaque regions of the mask. The resist is then developed, leaving a pattern of resist features that corresponds to the pattern of opaque features of the exposure mask (in the case of a positive resist), and the underlying wafer is selectively etched using the patterned resist to protect the wafer. FIG. 4 illustrates this ideal mode of operation. As shown in FIG. 4, resist features 4 are images of mask features 2. For simplicity, the projection lens between the mask and the wafer, and the image reduction effected by the projection lens are not shown in FIG. 4.
In operation, the optical lithographic stepper system effects stepwise relative movement of the exposure mask and wafer transverse to the axis of the system so that different fields of the wafer can be exposed through the mask. A field or “image field” is a region that is exposed without moving the wafer or the mask with respect to the lens; or in the case of a stepper-scanner instrument, a field is a region that is exposed in one, linear, continuous scanning motion of the wafer and mask stages. Stepper-scanners project a slit-shaped region, typically 26 mm by 8 mm, on the image plane (wafer). A field is exposed by scanning the slit-shaped image in a direction that is parallel to its short dimension. The maximum field size is on the order of 26 mm by 33 mm.
All imaging systems suffer from some amount of flare (an effect that mixes light from one part of the image with light from another part). Because flare represents imperfect behavior of an optical lithographic system, it is desirable to reduce the flare of the projection optics by good optics design and maintenance. However, some residual flare is inevitable even in the highest quality optics. This residual flare can be managed by taking flare into account in the design of the mask. The patterns on the mask can be compensated for flare and other optical imperfections. Compensating the mask for flare requires knowledge of the flare density function.
Flagello, D. et al., “Optimizing and Enhancing Optical Systems to Meet the Low k1 Challenge”, Proc. SPIE, vol. 5040, pp 139–150 (2003), discloses that scattered light in an optical lithography system may be measured using a reticle that is transparent except for opaque square pads of several different sizes. A photoresist detector is exposed through the reticle at several progressively increasing doses. The minimum exposure dose to clear each pad is determined.
Kirk, J. P., “Scattered Light in Photolithographic Lenses”, Proc. SPIE Vol. 2197, p. 566–572 (1994), discloses that flare may be measured by observing the extent to which an edge of the photoresist has receded from the corresponding edge of the geometric image of an opaque feature.
Conventional approaches to measuring flare by exposing a photoresist detector assume a functional form of the flare density function. Density functions that have been used in the prior art include Gaussian functions, a sum of Gaussian functions, and Lorentzian functions (see T. Brunner et al., Impact of resist blur on MEF, OPC and CD control, Proc. SPIE Vol 5377, p. 141–149, SPIE, Bellingham, 2004). The functional form of the density function is obtained from the measurements through regression.
F. Zach et al. “Aberration Analysis using Reconstructed Aerial images of Isolated Contacts on Attenuated Phase shift masks” disclose a double exposure method for determining aberrations of a exposure tool. In this disclosure a uniform first exposure is superimposed onto a second exposure that images a contact hole. Based on an analysis of the image intensity in the sidelobe aberrations can be extracted.
U.S. patent application Ser. No. 10/860,853, the entire disclosure of which is hereby incorporated by reference herein for all purposes, discloses the following techniques for measuring flare: (1) Use a reticle that is transparent except for opaque circular pads of several different sizes and measure the exposure dose to clear each resist pad (for positive photoresist) or the exposure dose to cover each resist hole (for negative photo-resist). (2) Use a reticle that is opaque except for one or more pin-holes. Measure the corresponding receding edge of positive photoresist or the expanding edge of negative photo-resist for several different exposure doses. (3) Use a reticle with one or more lines and measure the critical dimension (CD) change due to flare caused by a second exposure. In the above methods, the flare density function is obtained either directly (Methods 1 and 2) or through regression (Method 3).